1. Field of the Invention
This invention relates to suspension systems for automobiles and machines which receive mechanical shock, and more particularly to a method and apparatus for controlling shock absorbers.
2. Description of Related Art
Shock absorbers are used in connection with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb unwanted vibrations, shock absorbers are generally connected between the body and the suspension of the automobile. A piston is located within the shock absorber and is connected to the body of the automobile through a piston rod. Because the piston is able to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the suspension to the body. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the shock absorber.
In selecting the amount of damping that a shock absorber is to provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle, as well as the spring constant of the seat, tires, and the shock absorber. Vehicle handling is related to the variation in the vehicle's attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces are required to avoid excessively rapid variation in the vehicle's attitude during cornering, acceleration, and deceleration. Road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road holding ability, large damping forces are required when driving on irregular surfaces to prevent loss of contact between the wheels and the ground for an excessive period of time.
Because different driving characteristics depend on the amount of damping forces the shock absorber provides, it is often desirable to have a shock absorber in which the amount of damping forces generated by the shock absorber is adjustable. One method for selectively changing the damping characteristics of a shock absorber is disclosed in European Patent Application No. 85116585.2. In European Patent Application 85116585.2, a controller detects the distance between the body of the automobile and the front wheel so as to determine the contour of the surface. A rotary valve in each of the rear shock absorbers is then adjusted so that the rear shock absorbers are able to provide the desired amount of damping forces. Another method for selectively changing the damping characteristics of a shock absorber is disclosed in U.S. Pat. No. 4,600,215. In this reference, an ultrasonic sensor is used to determine the vertical displacement of the vehicle body from the road surface. The output from the sensor is compared to a reference signal which is responsive to vehicle speed. The result of the comparison provides an indication of the contour of the road surface which is used by the suspension system controller to adjust a rotary valve in each of the shock absorbers.
In those circumstances in which a rotary valve is used to control the flow of damping fluid between the upper and lower portions of the working chamber, the forces acting on the rotary valve are not generally balanced. For example, there is often a net upward force acting on the shutter sleeve due to the pressurized damping fluid inside the central bore of the shutter sleeve. This net upward force causes frictional forces to be developed between the shutter sleeve and the sensor housing. The actuator driving the rotary valve is therefore often required to oppose these relatively high frictional forces which tend to decrease the speed at which the rotary valve is able to change the damping characteristics of the shock absorber. In addition, the unbalanced forces acting on the rotary valve tend to reduce the expected lifetime of the components of the rotary valve.